At the present day, in order to transmit an image from a computer to a display, a means of transmitting a raster image with respect to each frame frequency is adopted. This means requires a large amount of transmission data, and therefore involves waste of transmission when a still image is being displayed.
As a measure for reducing the amount of transmission data, an image may be compressed into a file format such as JPEG and GIF, and then transmitted. However, it is necessary to have a processor which executes high-speed operations for compressing and decompressing the image with respect to each frame, thereby resulting in an increase in costs.
On the other hand, as a measure for reducing waste of transmission in displaying a still image, a display may be provided with a built-in frame memory which serves as a memory for storing a raster image to suspend the transmission of data when a still image is being displayed. This measure is effective especially for mobile information equipment and the like since it is also possible to reduce electric power consumption.
Reductions in electric power consumption and chip size are important for the display which is installed in mobile information equipment. Accordingly, it is desirable that an image stored in a memory should be displayed when displaying a still image, and that the capacity or size of the memory, which occupies a large proportion of chip size, should be small. By storing an image in a memory as just described, electric power consumption in data transmission can be reduced. Besides, The reduced demand of memory capacity means that chip size can be reduced.
In order to reduce memory capacity, image data may be compressed. However, according to a graphical data compression method utilizing a format such as JPEG and GIF, it is required to have an image processor for decompressing images, which counteracts the effect of reducing electric power consumption and chip size.
Another approach involves reducing the bit-plane number of a raster image. Here, the bit-plane number means the number n of bits of data representing the tone or gray level of a digital image which has been quantized by 2n. Examples of the methods for reducing the bit-plane number include the multi-valued dither method and the fixed threshold method. Details regarding these methods are described in “The New Image Electron Handbook” (Tokyo, Corona Publishing Co. Ltd., 1993), pp. 41-51. The multi-valued dither method and the fixed threshold method are different from the graphical data compression method utilizing a format such as JPEG and GIF in that compressed images do not require decompression.
FIG. 1 is a block diagram schematically showing the configuration of a conventional image processing apparatus. With reference to FIG. 1, a description will be given of an example of bit-plane compression of a raster image by the conventional multi-valued dither method, in which an RGB raster image with 6 bits per color component is transmitted from a computer, and an image display section displays an image with 6 bits per color component.
First, low order or least significant 2 bits of 6 bits per color component of a raster image 1 are transmitted to a comparator 12. A threshold value generator 11 generates a dither matrix based on ordered dithering, and outputs a 2-bit value, which is uniquely determined according to the pixel (X-Y coordinate values) of an input image, to the comparator 12.
The comparator 12 compares the low order 2 bits sent from the raster image 1 with the 2-bit value from the threshold value generator 11. When the value received from the threshold value generator 11 is larger than the low order 2 bits, the comparator 12 outputs “1” to a selector 13. Otherwise, the comparator 12 outputs “0” to the selector 13.
The selector 13 outputs high order or most significant 4 bits of the raster image 1 directly or a value obtained by subtracting 1 therefrom to a memory 2 based on the output value from the comparator 12. For an image with 4 bits per color component stored in the memory 2, the value of high order 2 bits of the 4 bits is attached as low order bits to the input 4-bit value by a bit addition section 14, and the image is output to an image display section 3 as a 6-bit image.
With this construction, the bit-plane number is reduced by the multi-valued dither method, and a pseudo image with 6 bits per color component is displayed.
Besides, Japanese Patent Publication No. HEI2-8493(B) (first prior art) has disclosed a digital signal processing apparatus which performs level-compression of a digitized input signal to transmit the signal and level-decompression of a compressed signal transmitted thereto, comprising circuits for detecting a maximum value of a digital signal, performing level-compression after adding a dither value to the digital signal based on the maximum value, performing level-decompression and subtracting the dither value from the digital signal, respectively.
With this construction, it is possible to obtain the output with smaller difference in a digitized audio signal being a one-dimensional signal.